1. Field of the Invention
The present invention relates to the field of cathode ray tube drivers. More specifically, the present invention relates to the field of cathode ray tub driver circuits with cathode current detection capabilities for high resolution display applications.
2. Related Art
Digital television formats such as high definition television (HDTV) and enhanced definition television (EDTV) provides for increased resolution in the television picture. The improvements in picture quality is accomplished via a new broadcasting standard that implements a wider bandwidth and higher frequencies than previously required for use with conventional television units. As such, supporting circuitry in televisions must be adapted to perform under the new broadcasting standard, such as the one supporting HDTV.
One component that needs adapting to the higher bandwidth and higher frequencies of the digital television formats is the cathode ray tube (CRT) driver used for amplifying a video input signal and generating a video output signal that drives a cathode in a television CRT.
In the past, CRT drivers for conventional televisions consisted of discrete circuits capable of easily handling the high power supplies necessary to run the CRT. The discrete circuits also could easily handle the lower frequencies and lower bandwidths of the analog broadcasting signal.
In contrast, discrete CRT drivers are incapable of driving the high resolution televisions capable of displaying digitally broadcast signals, such as HDTV and EDTV. The digital television standards require higher operating frequencies, higher speeds, and wider bandwidths. For example, the HDTV broadcasting standard requires double the operating frequency and triple the bandwidth of conventional television broadcasting signals. The detrimental effects of stray capacitance from components inherent in the CRT driver circuitry and corresponding printed circuit board (PCB) trace capacitance in discrete CRT drivers are increased in the higher frequencies and wider bandwidths of digital television broadcasting formats. This results in a television picture that is of poor quality.
In addition, it is desirous for the CRT driver circuit to have cathode current detection. Over time, the cathode in the electron gun of the CRT will deteriorate. This deterioration leads to a fluctuation in the cathode current which leads to a deterioration in color on the television display. The ability to measure the cathode current allows for recalibration of the electron gun in the CRT so that the magnitude of current generated by the cathode, in response to a test input signal, is consistent over time.
In one prior art example, the kinescope driver apparatus as disclosed in the White et. al patent (xe2x80x9cKinescope Driver Apparatus,xe2x80x9d U.S. Pat. No. 5,680,173), provides for cathode current detection as a measure of the voltage at an output node. FIG. 1 of the prior art is a schematic diagram of the White et. al circuit that illustrates a complementary push-pull emitter follower amplifier.
However, the White et. al circuit is incapable of accurately measuring the cathode current in its entirety as shown in FIG. 1 of the prior art. The output node 15 is coupled to a Class A emitter follower feedback control block 60 that is always on. As such, the current through the feedback control circuit 60 of FIG. 1 cannot be ignored and affects the cathode current measured at the output node 15.
For instance, the Q8 transistor in FIG. 1 is configured as an emitter follower. As such, its quiescent current is quite high, especially in a high-speed amplifier application (e.g., for HDTV). The higher speeds necessary to run a digital television application consumes more power and more current. Since the collector current of transistor Q8 is high, the base current of the Q8 transistor cannot be ignored. The higher the base current of the Q8 transistor, the higher the error for measuring cathode current in the Ik (cathode current) sense block 40. Thus, the cathode current measured in block 40 is not a true measurement of the cathode current since a large current through the feedback control block 60 exists.
Thus, there is a need for CRT driver circuits that are capable of handling the higher frequency and the higher bandwidths of the broadcast signal for digital television formats. Also, there is a need for CRT driver circuits that are capable of accurately measuring cathode current from the CRT.
Accordingly, a cathode ray tube (CRT) driver circuit suitable for high resolution display applications that also measures cathode current from a CRT is described. The present invention provides for a CRT driver circuit that is capable of handling the higher broadcast signal frequencies, higher operating speeds, and wider bandwidths of the broadcast signal for digital television formats. Additionally, the present invention provides the above accomplishment and is further capable of accurately measuring cathode current from the CRT.
Specifically, the present invention discloses a CRT driver circuit comprising a push-pull configuration comprising upper and lower stages of darlington paired transistors. The upper stage is comprised of two pairs of transistors arranged in darlington configuration. The two pairs of darlington configured transistors are coupled in series and form an upper prestage circuit and an upper output circuit. The lower stage is also comprised of two pairs of transistors arranged in darlington configuration. The two pairs of darlington configured transistors are coupled in series and a lower prestage circuit and a lower output circuit.
In the lower stage, the lower prestage circuit generates a video output signal in response to a video input signal that is amplified to drive a cathode electrode of a coupled CRT. In the upper stage, an upper prestage circuit of transistors drives a voltage divider for splitting a high voltage supply between the transistors in the lower prestage circuit.
In both the upper and lower stages of darlington paired transistors, upper and lower output stages of transistors are electrically active only during transient periods of the video input signal. As such, a cathode current from an effectively static test signal can be accurately measured from an output coupled to the lower prestage circuit.
The CRT driver circuit is formed as an integrated chip (IC chip). The IC chip formation reduces the internal stray capacitance and printed circuit board (PCB) external capacitance so that the CRT driver circuit can handle the higher frequencies, higher operating speeds, and wider bandwidths required for displaying broadcasting signals of digital television formats.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.